1. Field of the Invention
The present invention relates to a controller for a CRT display apparatus and, more particularly, to a controller for a CRT display apparatus of raster scan type.
2. Description of the Related Art
In a conventional medical field, X-ray photographs have been taken or X-ray image displayed on a fluorescent plate have been observed by physicians for diagnostic purposes, which results in exposure of the physicians to X-ray. In recent years, therefore, it is becoming a common measure to use an X-ray TV display apparatus so as to enable physicians to diagnose the object without any risk of exposure to X-ray. Such an X-ray display apparatus is usually constructed such as to record any image which the physician needs by photographing the image by a camera.
The CRT used in this apparatus is of a raster scan type, so that a problem is encountered that the image on the CRT photographed by the camera often contains clear scanning lines. To obviate this problem, it has been proposed to use a CRT display apparatus capable of displaying the image in raster erase mode in which the number of the raster scanning lines are increased by shifting them vertically to scan the gaps between the adjacent scanning lines, so that the scanning lines are obscured.
Computer-aided diagnostic X-ray system, referred to as CT (Computer Tomography), has also been known. In this type of diagnostic system, the image on the CRT tends to flicker by light. In some cases, therefore, the system is so designed that, when the image on the CRT is to be photographed by a camera, the image is displayed in so-called interlace mode in which the vertical scan is conducted such as to skip over every other scanning line, whereas, when photographing is not necessary, the image is displayed in non-interlace mode in which skipping of scanning line is not conducted.
A system called "digital subtraction system" has been developed in recent years and is expected to have a variety of use. This system is, for example, used for a X-ray TV system adapted to pick up any difference among a plurality of image data so as to display only the necessary image. This system is used to display images of various diagnostic objects including comparatively still objects, e.g., the head of a patient and comparatively moving objects such as the heart of a patient. In the medical diagnosis conducted with this type of system, therefore, the spatial resolution and temporal movement of the image have great significance. The most influential factor is the resolution of the image pickup device of the X-ray TV system. In particular, an improvement in the S/N ratio is very important. The improvement in the S/N ratio, however, is incompatible with the movement of the object to be diagnosed. In order to overcome this problem, it has been proposed to select a slow-speed scan mode or a high-speed scan mode, depending on whether the movement of the object to be diagnosed is large or not. Namely, when the object is comparatively still, the slow-speed scan mode is adopted so that the scanning is conducted at a comparatively low rate, e.g., 7.5 fields per second thereby increasing the S/N ratio. In this case, the object image is read with 1024.times.1024 pixels, and is displayed in interlace of 1024, thus attaining a high spatial resolution. In contrast, when the movement of the object is comparatively large, the high-speed scan mode is adopted in which the scanning is conducted at a comparatively high rate of, for example, 30 fields per second. In this case, the object image is read with 512.times.512 pixels, and is displayed in non-interlace of 512, thereby facilitating the observation of movement of the object. Thus, a single diagnostic system produces both an image signal for interlace mode display and an image signal for non-interlace mode display.
This causes a problem particularly in CRT display apparatus designed to display images selectively in interlace mode and non-interlace mode and having a function for displaying the image in raster erase mode mentioned before. Namely, in such CRT display apparatus, it is necessary to switch the operation mode between the interlace raster erase mode and the non-interlace raster erase mode. Conventionally, this switching of operation mode has been conducted manually by an operator through manipulating switches on the camera and the CRT.
This manual switching operation is rather laborious and complicated and, therefore, it has often been experienced that photographs of inferior resolution are obtained due to incorrect switching operation.